ond’s guess was substantiated over the course of the next seven years via the accidental discovery of the American fur trader Robert Gray and the more purposeful explorations of George Vancouver and Alexander Mackenzie working on behalf of British interests.

Actual geographic definition of the great western river finally began to supplant hopeful theorizing, but only barely. The Columbia River’s mouth was established at the 46th parallel, a propitious finding since that seemed to match the envisioned headwaters of the Missouri, as delineated by Pond and others.

Alexander Mackenzie complicated this core article of geographic faith with the putative discovery of a northern fork of the Columbia River in present British Columbia. Still, the mythic ideal of two great western rivers with adjoining sources held sway as the last great hope for the fabled Northwest Passage.

Have fun exploring the details of these maps by taking advantage of the DjVu map viewer to see high resolution maps online.